1. Field of the Invention
The present invention is directed to scanning probe microscopy in fluid, and more particularly, to an apparatus for cleaning the fluid exposed surface of a scanning probe microscope while protecting fluid sensitive components.
2. Description of Related Art
Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are devices which typically employ a probe having a tip and which cause the tip to interact with the surface of a sample with low forces to characterize the surface down to atomic dimensions. Generally, the probe is introduced to a surface of a sample to detect changes in the characteristics of a sample. By providing relative scanning movement between the tip and the sample, surface characteristic data can be acquired over a particular region of the sample, and a corresponding map of the sample can be generated.
A typical AFM system is shown schematically in FIG. 1. An AFM 2 employs a probe device 3 including a probe 3 having a cantilever 4. A scanner 5 generates relative motion between the probe 3 and a sample 6 while the probe-sample interaction is measured. In this way, images or other measurements of the sample can be obtained. Scanner 5 is typically comprised of one or more actuators that usually generate motion in three mutually orthogonal directions (XYZ). Often, scanner 5 is a single integrated unit that includes one or more actuators to move either the sample or the probe in all three axes, for example, a piezoelectric tube actuator. Alternatively, the scanner may be a conceptual or physical combination of multiple separate actuators. Some AFMs separate the scanner into multiple components, for example an XY actuator that moves the sample and a separate Z-actuator that moves the probe. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography or some other property of the sample as described, e.g., in Hansma et al. U.S. Pat. No. RE 34,489; Elings et al. U.S. Pat. No. 5,266,801; and Dings et al. U.S. Pat. No. 5,412,980.
Notably, scanner 5 often comprises a piezoelectric stack (often referred to herein as a “piezo stack”) or piezoelectric tube that is used to generate relative motion between the measuring probe and the sample surface. A piezo stack is a device that moves in one or more directions based on voltages applied to electrodes disposed on the stack. Piezo stacks are often used in combination with mechanical flexures that serve to guide, constrain, and/or amplify the motion of the piezo stacks. Additionally, flexures are used to increase the stiffness of actuator in one or more axis, as described in U.S. Ser. No. 11/687,304, filed Mar. 16, 2007, entitled “Fast-Scanning SPM Scanner and Method of Operating Same.” Actuators may be coupled to the probe, the sample, or both. Most typically, an actuator assembly is provided in the form of an XY-actuator that drives the probe or sample in a horizontal, or XY-plane and a Z-actuator that moves the probe or sample in a vertical or Z-direction.
In a common configuration, probe 3 is often coupled to an oscillating actuator or drive 16 that is used to drive probe 3 to oscillate at or near a resonant frequency of cantilever 4. Alternative arrangements measure the deflection, torsion, or other characteristic of cantilever 4. Probe 3 is often a microfabricated cantilever with an integrated tip 7.
Commonly, an electronic signal is applied from an AC signal source 18 under control of an SPM controller 9 to cause actuator 8 (or alternatively scanner 5) to drive the probe 3 to oscillate. The probe-sample interaction is typically controlled via feedback by controller 9. Notably, the actuator 8 may be coupled to the scanner 5 and probe 3 but may be formed integrally with the cantilever 4 of probe 3 as part of a self-actuated cantilever/probe.
Often, a selected probe 3 is oscillated and brought into contact with sample 6 as sample characteristics are monitored by detecting changes in one or more characteristics of the oscillation of probe 3, as described above. In this regard, a deflection detection apparatus 25 is typically employed to direct a beam towards the backside of probe 3, the beam then being reflected towards a detector 11, such as a four quadrant photodetector. The deflection detector is often an optical lever system such as described in Hansma et al. U.S. Pat. No. RE 34,489, but may be some other deflection detector such as strain gauges, capacitance sensors, etc. The sensing light source of apparatus 10 is typically a laser, often a visible or infrared laser diode. The sensing light beam can also be generated by other light sources, for example a He—Ne or other laser source, a superluminescent diode (SLD), an LED, an optical fiber, or any other light source that can be focused to a small spot. As the beam translates across detector 11, appropriate signals are processed by a signal processing block 13 (e.g., to determine the RMS deflection of probe 3). The interaction signal (e.g., deflection) is then transmitted to controller 9, which processes the signals to determine changes in the oscillation of probe 3. In general, controller 9 determines an error at Block 14, then generates control signals (e.g., using a PI gain control Block 32) to maintain a relatively constant interaction between the tip and sample (or deflection of the lever 4), typically to maintain a setpoint characteristic of the oscillation of probe 3. The control signals are typically amplified by a high voltage amplifier 16 prior to, for example, driving scanner 5. For example, controller 9 is often used to maintain the oscillation amplitude at a setpoint value, AS, to insure a generally constant force between the tip and sample. Alternatively, a setpoint phase or frequency may be used. Controller 9 is also referred to generally as feedback where the control effort is to maintain a constant target value defined by setpoint.
A workstation 17 is also provided, in the controller 9 and/or in a separate controller or system of connected or stand-alone controllers, that receives the collected data from the controller and manipulates the data obtained during scanning to perform data manipulation operating such as point selection, curve fitting, and distance determining operations. The workstation can store the resulting information in memory, use it for additional calculations, and/or display it on a suitable monitor, and/or transmit it to another computer or device by wire or wirelessly. The memory may comprise any computer readable data storage medium, examples including but not limited to a computer RAM, hard disk, network storage, a flash drive, or a CD ROM.
AFMs may be designed to operate in a variety of modes, including contact mode and oscillating mode. Operation is accomplished by moving the sample and/or the probe assembly up and down relatively perpendicular to the surface of the sample in response to a deflection of the cantilever of the probe assembly as it is scanned across the surface. Scanning typically occurs in an “x-y” plane that is at least generally parallel to the surface of the sample, and the vertical movement occurs in the “z” direction that is perpendicular to the x-y plane. Note that many samples have roughness, curvature and tilt that deviate from a flat plane, hence the use of the term “generally parallel.” In this way, the data associated with this vertical motion can be stored and then used to construct an image of the sample surface corresponding to the sample characteristic being measured, e.g., surface topography. In one practical mode of AFM operation, known as TappingModem™ AFM (TappingModem™ is a trademark of the present assignee), the tip is oscillated at or near a resonant frequency of the associated cantilever of the probe, or harmonic thereof. A feedback loop attempts to keep the amplitude of this oscillation constant to minimize the “tracking force,” i.e., the force resulting from tip/sample interaction, typically by controlling tip-sample separation. Alternative feedback arrangements keep the phase or oscillation frequency constant. As in contact mode, these feedback signals are then collected, stored and used as data to characterize the sample.
Regardless of their mode of operation, AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum by using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers fabricated using photolithographic techniques. Because of their resolution and versatility, AFMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research. Note that “SPM” and the acronyms for the specific types of SPMs, may be used herein to refer to either the microscope apparatus or the associated technique, e.g., “atomic force microscopy.”
One particular use of AFMs is the imaging of samples in a fluid/liquid medium. For example, a TappingMode™ AFM may be used for the visualization of supported lipid bilayers or adsorbed single polymer molecules under liquid medium. For the imaging of samples supported in a fluid medium, some AFM components will reside in the fluid or liquid medium. To maintain the integrity of the sample, i.e., preventing contamination of the sample, the exposed surface of those components must be substantially free of any contaminants. Accordingly, before imaging, or otherwise capturing data, of a sample supported in the fluid medium, the exposed surfaces must be cleaned. In the most general terms, cleaning the exposed surfaces, for instance, the scanner, involves exposing the surfaces to one or more cleaning agents, i.e., soap and/or detergent, as well as a scrubbing tool, i.e., brush. One of the challenges with cleaning the surfaces that are loaded into the fluid medium is ensuring that fluid sensitive components are not exposed to the cleaning fluids. Hence, conventional AFMs have the fluid sensitive components, e.g., actuator elements and sensor elements, encased in a sealed housing that is separate from the cantilever holder, which is exposed to the fluidic sample and therefore should be cleaned prior to sampling. Having the cantilever holder separable from the sealed housing requires a mounting interface, which adds to the mass of the scanner and reduces the stiffness of the scanner.
As described in U.S. Ser. No. 11/687,304 high resolution at higher scan rates is achievable by increasing the fundamental resonant frequency of the tip scanner. Two of the numerous ways in which the fundamental resonant frequency of a tip scanner can be increased is scanner mass and scanner stiffness. More particularly, decreasing the mass of the scanner increases the fundamental resonant frequency of the tip scanner. Similarly, flexures, for example, can increase the stiffness of the scanner and consequently increase the scanner's fundamental resonant frequency. Thus, while a cantilever holder separable from the “fluid proof” encasing for the fluid sensitive components eases the challenges associated with cleansing the cantilever holder and the exposed surface(s) of the scanner; ultimately, such a construction decreases the fundamental resonant frequency of the scanner and therefore reduces the scan rate for the scanner. On the other hand, integrating the cantilever holder with the housing for the scanner electronics risks exposure of the fluid sensitive components to the cleaning agents during cleaning of the scanner.
One proposed solution is to enclose the scanner electronics, e.g., piezo actuators and strain gauges, in a housing to which the cantilever holder is attached. This integration of the electrical components and the cantilever holder provides a less massive and more stiff scanner, as described in U.S. Ser. No. 13/068,052 the disclosure of which is incorporated herein. To protect the electrical components from exposure to the cleaning agents and/or solutions, the components could be encased in an elastic sealant. However, given the highly sensitive nature of AFMs, the elastic sealant may negatively affect performance. Additionally, it is possible that the sealant could degrade over time as it is exposed to the cleaning agents and/or solutions. An improved solution was desired.